Q.Rapid multiplication of which of the following groups of organisms make the sea appear red and cause red tide?

Dinoflagellates

Imagine a microscopic armoured tank, spinning through the ocean. That's the basic picture of a dinoflagellate. The name comes from the Greek dinos (whirling) and Latin flagellum (whip) — a reference to the spinning motion they make as they swim.

These are single-celled protists, mostly marine, and they are a big deal in the ocean ecosystem. Some are photosynthetic, some are predators, and some do both. But the first thing you notice under a microscope is their armour.

The Armour and the Whips

Dinoflagellates have a rigid outer covering made of cellulose plates — like a suit of armour. This is called the theca. The plates fit together in patterns that are used to identify different species.

They have two flagella (whip-like tails), but they are arranged in a unique way:

• One flagellum wraps around the cell in a groove called the cingulum (like a belt). Its beating makes the cell spin.
• The other flagellum trails behind in a groove called the sulcus. It provides forward thrust.

The result is a spinning, forward-moving motion — like a top that also moves ahead. This is why they are called "whirling flagellates."

The Red Tide Problem

Some dinoflagellates contain red pigments. When conditions are perfect — warm water, lots of nutrients, calm seas — they can multiply explosively, creating a bloom that turns the water reddish-brown. This is a red tide.

The danger is not the colour. Many red-tide species release potent neurotoxins that affect the nervous system. These toxins can:

• Kill fish and marine mammals directly.
• Accumulate in shellfish (clams, mussels, oysters) that filter-feed on the dinoflagellates.
• Cause paralytic shellfish poisoning in humans who eat the contaminated shellfish — a serious medical emergency.

The Precise Statement